Hi Chris
As far as I can tell the Boxster gearbox looks like an Audi 01A gearbox so is should bolt up to the Audi V8, but I can't confirm that.
I can however tell you that a Audi V8 will bolt onto an A4 or A6 2wd gearbox because I have done it. They are available as either 5 or six speed and come up on eBay all the time.
I have got a 1995 -2000 4.2 A8 engine (ABZ) bolted to a 01X 6 speed A4 TDi gearbox and it goes together no problem.
You will probably find the A4/A6 gearbox easier to find than the Boxster unit.
The engine length you quote sounds like one of the newer cam chain engines fitted to the S4, the older units with the cam belt are a bit longer. The weight is about right as I measured mine at 180Kg without oil and water.
The approx sizes of the Audi ABZ V8 engine and gearbox are: -
Total length (Audi ABZ engine + 01X gearbox) = 1200mm
Length of engine (front pulley to bellhousing face) = 490mm
Length of 01X gearbox (bellhousing to rear casing) =710mm
Max width of engine (across exhaust manifolds) = 760mm
Max height of engine (sump to idle air valve) = 660mm
Drive shaft position (front pulley to drive flange) =690mm (or from gearbox rear casing = 510mm)
Hilly